1. Field of the Invention
This invention relates in general to digital quartz stabilized FM discriminators.
2. Description of the Prior Art
PLL (phase locked loop) demodulators which normally regulate a VCO (voltage controlled oscillator) with a low pass filter in a manner such that the VCO frequency is equal to the input frequency where the regulating voltage is connected to the VCO corresponds to the LF voltage have the disadvantage that due to their temperature and voltage dependency the VCOs are difficult to stabilize. This requires that the VCO be adjusted. The curve of the VCO corresponds to the demodulator curve which means that nonlinearities produce a distortion factor.
When the VCO is operated without a low pass filter, a digital on/off regulator is obtained, the VCO produces only two frequencies which correspond to the lowest and highest demodulatable frequencies. The pulse duty factor corresponds to the ratio of the input frequency to the two possible VCO frequencies.